Oven

ABSTRACT

An oven for curing coating material applied to metal sheets (15) has a heating section (17), a cooling section (18) and an incinerator (19) for burning fumes from the coating material. The oven also as a conveyor for carrying sheets through it. The oven is associated with a sheet feeder (11), a coating apparatus (12), and a sheet stacker (13). In one embodiment, a conveyor (16) comprises an air bed (30) and two series of drive members (35) mounted on two endless chains (36). Air is ejected through a matrix of slots in the air bed (30) and thereby creates a cushion for supporting the sheets (15). The air is ejected in a direction which urges the trailing edges of the sheets into engagement with the drive members (35) and the drive members (35) propel the sheets through the oven. In another embodiment, a conveyor (200) comprises two series of drive members (205, 206) mounted on spaced apart chain (208, 209) and arranged in pairs for engaging the trailing and leading edges of metal sheets. The sheets (15) are supported on a cushion of air ejected from an air bed (201).

TECHNICAL FIELD

This invention relates to an oven for curing coating material applied tometal sheets. This invention is particularly, but not exclusively,concerned with an oven for curing coating material applied to metalsheets which are used, for example, in the manufacture of metal cans,ends for metal cans, metal boxes and metal trays. The term "metalsheet", as used herein, means a sheet formed wholly or partly frommetal.

BACKGROUND ART

As a preliminary step in the manufacture of metal cans from a metalsheet, one or more layers of coating material are applied to the sheet.These layers may provide functional protective features and may also beapplied for decorative purposes. Examples of such coating material aresizes, pigmented coatings, inks, lacquers and varnishes. For example,where a metal sheet is to be used in the manufacture of bodies for threepiece metal food cans, a layer of lacquer may be applied to one surfaceof the sheet. By way of another example, where a metal sheet is to beused in the manufacture of bodies for aerosol cans, a layer of lacquermay be applied to one surface of the sheet and layers of size, whitecoating, inks, and varnish may be applied, in turn, to the othersurface. Where a metal sheet is being used in the manufacture of canbodies, after it has received the required number of layers of coatingmaterial, small rectangular workpieces are cut from the sheet and theseare then formed into can bodies. Where a sheet is used in themanufacture of other components, for example, ends for can bodies,workpieces of suitable shape and size are cut from the metal sheet andthen formed into the desired components.

After a layer of coating material has been applied to a metal sheet, thecoating material is cured by passing the sheet through an oven. Aconventional oven for metal sheets comprises a heating section, acooling section and a wicket conveyor. A wicket conveyor comprises aseries of wickets which are mounted at spaced intervals on endlesschains. Each wicket is arranged to carry a sheet through the oven in asubstantially vertical position with the coated and wet surface of thesheet out of contact with the wicket. When an oven is in operation, thewicket conveyor carries sheets in turn firstly through the heatingsection and then through the cooling section. In the heating section,heated air is passed over the sheets. Typically, in the heating sectionthe temperature of each sheet is raised to a temperature in the range of120° C. to 230° C., the transit times of the sheets through the heatingand cooling sections are 10 to 20 minutes and 3 to 5 minutes,respectively, and 5000 sheets an hour enter the oven. Thus, it is normalfor about 1600 sheets to be present in an oven. The temperature andtransit times may be varied in accordance with the specific coatingmaterial being used.

There are various problems associated with the conventional oven. Duringoperation, volatile components from the coatings condense on thewickets. In other circumstances, the wickets may corrode. Thus, thewickets are a source of dirt. If such dirt is carried by air currents inthe oven and transferred to a layer of coating material while it isstill wet, the coating material is spoilt. In order to reduce thepresence of dirt, the wickets are cleaned periodically and this resultsin a loss of production time. As a metal sheet is carried through theheating section by a wicket, there are periods when the wicket and metalsheet are at different temperatures. Consequently, heat transfer betweenthe wicket and the metal sheet can interfere with the curing process andthis leads to marks in the cured layer of coating material. The wicketsare prone to vibration and the vibration together with sheet movementsinduced by air currents can cause abrasion damage on the sides of thesheets resting on the wickets. Occasionally, uneven heating causes asheet to buckle. Such buckling together with movement induced by aircurrents can cause contact between a sheet and a neighboring wicket.Such contact causes damage to the coating. Because one surface of ametal sheet rests against a wicket as it passes through the oven, it ispossible to cure only a single surface of the sheet during each passage.While the arrangement of coated sheets supported on their lower edges onwickets has proved satisfactory for many years (apart from the problemsjust mentioned), there is now a requirement to coat thinner metal sheetshaving thicknesses less than 0.2 mm. Such sheets tend to bend whensupported on their edges and such bending could cause contact between asheet and a neighboring wicket.

DISCLOSURE OF THE INVENTION

It is an object of this invention to provide a new or improved oven forcuring coating material applied to workpieces in the form of sheets inwhich the above mentioned disadvantages are overcome or reduced.

According to this invention, there is provided an oven for curingcoating material applied to metal sheets, said oven comprising a heatingsection for applying heat to metal sheets and a conveyor for conveyingmetal sheets through the heating section, in which the conveyorcomprises an elongate bed extending through the heating section, aplenum chamber located beneath the bed, a matrix of jets formed in thebed for supplying gas from the plenum chamber to the space above the bedso as to form a gas cushion, the gas cushion being capable of supportingmetal sheets, and means for propelling sheets along the bed.

Because the sheets are propelled through the oven on a gas cushion, theuse of wickets is eliminated. Consequently, the various problemsassociated with wickets do not exist.

In one arrangement of the conveyor, the propelling means comprises atleast one endless line extending through the heating section, means forimparting motion to said at least one endless line and a series of drivemembers mounted on said at least one endless line at spaced apartintervals, said drive members being arranged to engage the trailingedges of metal sheets passing along the conveyor, and the jets beingarranged to eject gas in a direction which urges the metal sheets intoengagement with the drive members.

In another arrangement, the propelling means comprises at least oneendless line extending through the heating section, means for impartingmotion to said at least one endless line, a first series of drivemembers mounted on said at least one endless line at spaced apartintervals, said first series of drive members being arranged to engagethe leading edges of metal sheets passing along the conveyor, and asecond series of drive members mounted on said at least one endless lineat spaced apart intervals, said second series of drive members beingarranged to engage the trailing edges of metal sheets passing along theconveyor.

Conveniently, the heating section includes a second plenum chamberhaving a lower wall positioned above the bed of the conveyor, means forsupplying heated gas to the second plenum chamber, and means forinjecting gas from the second plenum chamber to the space below saidlower wall.

Desirably, the injecting means comprises a set of slots in said lowerwall, each slot being oriented so that its projection onto the bed ofthe conveyor is inclined to the direction of travel of workpiece alongthe conveyor by an angle less than 90° C.

By orienting the slots in this manner, there is less tendency for thegas injected from the second plenum chamber to induce vibrations in ordisturbance of the sheets.

The oven as set forth in the preceding paragraphs may be combined with acoating apparatus, the coating apparatus being arranged to apply a layerof coating material simultaneously to both surfaces of a sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will now be described in more detail, by way of example,with reference to the drawings in which:

FIG. 1 is a diagrammatic side view of an oven embodying this inventiontogether with an associated sheet feeder, coating apparatus and sheetstacker;

FIG. 2 is a diagrammatic side view of part of the heating section of theoven of FIG. 1 and shows one embodiment of a conveyor;

FIG. 3 is a longitudinal sectional view of part of the conveyor used inthe oven of FIG. 1;

FIG. 4 is a diagrammatic view of drive members used in the conveyor;

FIG. 5 is a view on line 5--5 of FIG. 2;

FIG. 6 is a fragmentary view, partly in section, of a modified drivemember for the conveyor;

FIG. 7 is a diagrammatic side view of the front part of the oven of FIG.1 showing an alternative embodiment of the conveyor;

FIG. 8 is a diagrammatic side view of the rear part of the oven of FIG.1 provided with the alternative conveyor;

FIG. 9 is a diagrammatic side view of part of the heating section of theoven of FIG. 1 provided with the alternative conveyor;

FIG. 10 is a view on line 9--9 of FIG. 9;

FIG. 11 is a fragmentary plan view of the air bed of the oven of FIG. 1when provided with the alternative conveyor and showing a pair of drivemembers and associated chains;

FIGS. 12(a) to 12(e) are diagrams illustrating the engagement of theleading edge of a metal sheet with a drive member of the alternativeconveyor;

FIGS. 13(a) to 13(e) are diagrams illustrating the engagement of thetrailing edge of a metal sheet with a drive member of the alternativeconveyor;

FIG. 14 is a cross-sectional view of the heating section of the oven ofFIG. 1;

FIG. 15 is a circuit diagram of the arrangement for supplying heated airto the heating section of the oven of FIG. 1;

FIG. 16 is a circuit diagram of the arrangement used for supplying coolair to the cooling section of the oven of FIG. 1;

FIG. 17 shows graphs comparing the curing cycle of conventional andrapid cure materials;

FIG. 18 is a block diagram of a modification to the heating section ofthe oven of FIG. 1 in which coating material is cured by inductioncoils;

FIG. 19 is a perspective view of part of the heating section of the ovenof FIG. 1 and shows the induction coils of FIG. 17;

FIG. 20 is a block diagram of another arrangement of induction coils foruse in the heating section of the oven of FIG. 1; and

FIG. 21 is a block diagram of a further arrangement of induction coilsfor use in the heating section of the oven of FIG. 1.

MODES OF CARRYING OUT THE INVENTION

Referring now to FIG. 1, there is shown an oven 10 together with anassociated sheet feeder 11, coating apparatus 12, and sheet stacker 13.The sheet feeder 11 and sheet stacker 13 are of well known design.

In operation, metal sheets 15 are supplied by the sheet feeder 11 to thecoating apparatus 12. The sheets 15 are then carried by a conveyor, notshown in FIG. 1, from the coating apparatus 12 through the oven 10 andthey are then stacked by the sheet stacker 15. Two embodiments of theconveyor will be described below. The conveyor supports the sheets on acushion of air and carries them through the oven 10 in a substantiallyhorizontal attitude.

The oven 10 further comprises a heating section 17, a cooling section 18and an incinerator 19. In the heating section 17, heated air is ejectedfrom a plenum chamber 20 so as to raise the metal sheets 15 to therequired temperature. The fumes from the coating material are burnt inthe incinerator 19. In the cooling section 18, cool air is injected froma pair of plenum chambers 21, 22. By way of modification, the heatingand cooling sections may have additional plenum chambers.

One embodiment of a conveyor 16 and the plenum chamber 20 will now bedescribed with reference to FIGS. 2 to 5.

Referring now to FIGS. 2, 3 and 4, the conveyor 16 includes an elongateair bed 30 which extends between the coating apparatus 12 and the sheetstacker 13. In the heating section, the air bed 30 is mounted on aplenum chamber 31 which has an inlet 32. In the cooling section, the airbed 30 is mounted on a separate plenum chamber.

A matrix of jets 33 are formed in air bed 30. In operation, pressurizedair is supplied to the plenum chamber 31 and the jets 33 inject air intothe space above air bed 30 so as to form a cushion of air for supportingthe sheets 15. In order to propel the sheets 15 through the oven 10, theconveyor 16 includes two series of drive members 35. Each series ofdrive members 35 is mounted on a respective endless chain 36. Theendless chains 36 are spaced apart laterally and the drive members 35pass through grooves formed in the air bed 30. In FIGS. 2, 3 and 4, onlyone series of drive members 35 and a single endless chain 36 are shown.As shown in FIG. 4, each of the endless chains 36 passes over a pair ofpulleys 37, 38 adjacent the coating apparatus 12 and over a further pairof pulleys 39, 40 adjacent the sheet stacker 13. The two pulleys 40 aremounted on a common shaft which is driven by an electric motor 41 via adrive chain 42 and drive pulley 43.

As shown in FIG. 3, the jets 33 are inclined rearwardly with respect tothe direction of travel of sheets 15 along the conveyor. Consequently,the air ejected by jets 33 urges the sheets 15 against the drive members35, which engage the trailing edges of sheets 15. With this arrangement,it has also been found that the sheets 15 are carried through the oven10 in a stable manner and at spaced intervals which are controlled bythe spacing of the drive members 35.

The air bed 30 may be horizontal or, alternatively, slope upwardly withprogression through the oven 10. If the air bed 30 slopes upwardly,contact between the sheets 10 and drive members 35 is assisted bygravity.

Referring now to FIGS. 2 and 5, the plenum chamber 20 has an inlet 51and a lower wall 52. Heated air is supplied to the inlet 51 and this airis ejected through slots 53 formed in lower wall 52. As may be seen fromFIG. 5, each of the slots 53 is oriented so that its projection onto thebed 30 is inclined by approximately 45° to the direction of travel ofthe sheets 15. With the slots 53 oriented in this manner, it has beenfound that there is no significant tendency for the air ejected throughthe slots to induce vibrations in or disturbance of the sheets 15. Asmay be seen in the present example, the slots 53 are arranged in pairs,the two slots of each pair being arranged to present a V-shape orchevron profile. By way of modification, the slots 53 may be replaced byperforations in lower wall 52.

The plenum chambers 21, 22 have a similar construction to that of plenumchamber 20.

Referring now to FIG. 6, there is shown a modification to the drivemembers 35. FIG. 6 shows a single modified drive member 54 together witha short length of the drive chain 36. The modified drive member 54comprises an aluminium body 55 having a V-shaped cutout 56 whichprovides positive location of metal sheets. Three transverse bores areformed in the body 55 between the apex of the cut-out 56 and theopposite side of the body 55. Each of these bores contains a bar magnet,one of which is shown and designated by reference numeral 57. Themagnets are provided with a steel keeper 58. When the drive member 54 ispropelling a steel sheet, the magnets 57 help to keep the steel sheet incontact with the drive member 54.

An alternative and preferred embodiment of a conveyor 200 for the oven10 together with the plenum chamber 20 will now be described withreference to FIGS. 7 to 13.

Referring to FIGS. 7 to 11 the conveyor 200 includes a horizontal airbed 201 which extends from just inside the entrance of heating section17 to just inside the exit of cooling section 18. In the heating section17, the air bed 201 is mounted on a plenum chamber 202 which has aninlet 203. In the cooling section 18, the air bed is mounted on separateplenum chamber.

A matrix of upwardly directed jets 204 are formed in the air bed 201. Inoperation, pressurized air is supplied to the plenum chambers and thejets 204 inject air into the space above the air bed 200 so as to form acushion of air for supporting the sheets 15.

In order to propel the sheets 15 through the oven 10, the conveyor 200includes two series of drive members 205, 206. Each of the drive members205, 206 extends across the entire width of the air bed 201 and is ofV-shape in cross-section. The drive members 205, 206 are arranged inpairs. In each pair, the drive member 205 is arranged to engage thetrailing edge of a sheet 15 while its leading edge is engaged by a drivemember 206. Thus, in each pair, the drive members 205, 206 are spacedapart by a distance equal to the length of the sheets 15.

The ends of the drive members 205 are mounted on a pair of laterallyspaced apart chains 208. Similarly, the drive members 206 are mounted ona pair of laterally spaced apart chains 209. The ends of the drivemembers 205 are cranked to permit them to bridge the chains 209. Formost of their length, the chains 208, 209 are parallel so that only oneof them is visible in the drawings (except for FIG. 11 in which all fourchains are visible).

As shown in FIG. 7, beneath the entrance of the heating section, thechains 208, 209 pass over a single pulley 212. The chains 208, 209 thenpass over a pulley 213 before entering the heating section 17.

As shown in FIG. 8, after passing through the exit of the coolingsection 18, the chains 208 pass over a pulley 214 and the chains 209pass over a pulley 216. After passing over pulleys 214, 216, all fourchains 208, 209 pass over a common pulley 217. Between the pulleys 217and 212, the chains 208, 209 pass over a series of pulleys, not shown.

In order to drive the chains 208, 209, there is provided a combinedmotor and gear box unit 222 which has a drive pulley 223. The drivepulley 223 drives pulleys 214, 216 respectively via belts 224, 225. Thepulleys 214, 216 are driven at the same speed and, consequently thechains 208, 209 also travel at the same speed. The belt 224 may bedisconnected from the motor of unit 222 to adjust the spacing betweeneach pair of drive members 205, 206. The chains 208 are longer than thechains 209 by a distance equal to the spacing between two adjacent drivemembers 205 or 206.

Referring now to FIGS. 7 and 8, between the coating apparatus 12 and thepulley 213, the metal sheets are transported on the upper run of anendless belt conveyor 230. Between the pulley 213 and the beginning ofthe air bed 201, the central parts of the sheets are supported on theupper run of an endless belt conveyor 231. The upper run of the endlessbelt conveyor is slightly higher than the apexes of the drive members205, 206 while the apexes of the drive members 205, 206 are slightlyabove the upper run of the endless belt conveyor 231. The metal sheetsare carried from the pulley wheels 214, 216 to the sheets stacker 15 byan endless belt conveyor 233. The upper run of the endless belt conveyoris slightly below the apexes of drive members 205, 206. A furtherendless belt conveyor may be provided to support the sheets 15 betweenthe end of the air bed 201 and the pulleys 214, 216.

The engagement of a sheet 15 by a pair of drive members 205, 206 isillustrated in FIGS. 12(a) to 13(e). These figures view the oven 10 fromthe opposite side to that of FIG. 1. The endless belt conveyor 230 runsat a slightly greater speed than the chains 208, 209. Consequently, asshown in FIGS. 12(a) to 12(e), the leading edge of the sheet 15 isbrought firmly into engagement with the drive member 206.

The plenum chamber 20 is shown again in FIGS. 9 and 10. By way ofmodification, when the oven 10 is provided with the conveyor 200 ofFIGS. 7 to 11, the slots in the lower wall 52 of plenum chamber 20 maybe transverse, that is inclined at 90° to the direction of travel of thesheets.

Referring now to FIG. 14, there is shown a cross-section of the heatingsection 17. As shown, the heating section has a wall 234 formed ofinsulating material which encloses the plenum chambers 20 and 31 or 202.Although not shown, the wall 234 has panels which may be removed topermit access to the interior of the heating section 11. The plenumchamber 20 is pivotally mounted to permit movement between its normaloperating position, indicated by reference numeral 235, and a raisedposition, indicated by reference numeral 236. The plenum chamber 20 ismoved to its raised position to permit access to the air bed. The plenumchambers 20 and 31 or 202 receive air through ducts 237, 238.

Referring now to FIG. 15 there is shown the circuit for supplying heatedair to the plenum chambers 20 and 31 or 202. The circuit includes an airinlet 61, three fans 62, 66, 67, two heat-exchangers 63, 65, acombustion chamber 64, two by-pass control valves 68, 69 and an outlet70. The components 61 to 70 form the incinerator 19.

Air from inlet 61 passes through fan 62 and the secondary side of heatexchanger 63 to the inlet of plenum chamber 31 or 201. Exhaust air fromthe heating section 17 is divided into two parts. The first part issupplied to the inlet of combustion chamber 64. The products ofcombustion pass through the primary sides of heat exchangers 65 and 63and the fan 67 to the outlet 70. The second part passes through thesecondary side of heat exchanger 65 and the fan 66 to the inlet ofplenum chamber 20.

Thus, in the incinerator 19, the heat of combustion from the fumes isused to heat the air which is injected into both plenum chambers.

Referring now to FIG. 16, there is shown the circuit for supplying coolair to the plenum chambers 21, 22, 88 in the cooling section 18. Plenumchamber 88 is located beneath air bed 30 in the cooling section. Thiscircuit includes a pair of inlets 80, 81, an outlet 82, a pair offilters 83, 84, and three fans 85, 86, 87.

Air from inlet 80 passes through filter 83 and fan 85 into the plenumchamber 88. Similarly, air from inlet 81 passes through filter 84 andfan 87 into plenum chambers 21, 22. Air is withdrawn from the coolingsection 18 by fan 86 and ejected through outlet 82.

For some applications it may be desired to cure coating material in agaseous atmosphere other than air. For example, it may be desired tocure coating material in an atmosphere of nitrogen or an inert gas. Inorder to cure coating material in an atmosphere of a particular gas, thegas is supplied to plenum chambers 20, 21, 22, 31 or 202 and 88 andincinerator 19 is replaced with a heater.

When curing conventional coating materials in a conventional oven, thetemperature of each sheet is raised over a period of five or six minutesto a temperature in the range of 120° C. to 230° C. and each sheetspends approximately 10 to 20 minutes in the heating section and 3 to 5minutes in the cooling section. The first part of the curing cycle of aconventional coating material is shown in FIG. 17 by graph C.

For a particular oven, the rate of entry of sheets into the oven islimited by the capacity of the oven and the total time spent by eachsheet in the oven. It is normal for approximately 5000 sheets/hour toenter a conventional oven. Because sheets enter the oven of FIGS. 1 to16 in a horizontal attitude, it has a much lower capacity than aconventional oven of the same length. Consequently, if it were used tocure conventional coating material, the maximum rate of entry of sheetsinto the oven would be much lower than the rate achievable with aconventional oven of the same length.

There have been developed so called "rapid cure" coating materials. Theapplications for such coating materials presently include coil coatingand coating of can bodies after they have been formed into the desiredshape. These coating materials include lacquers, sizes, pigmentedcoatings, inks and varnishes. They are cured by rapid elevation totemperatures in the region of 180° C. or higher followed immediately bycooling. The curing cycle for a typical rapid cure coating material isshown by graph R in FIG. 17. As may be seen, the coating material iselevated to a peak temperature of 200° C. in about 8 seconds. It is thencooled to 40° C. in a further period of about 16 seconds. When thetemperature has been reduced to below 40° C., the coating material is nolonger tacky and can be handled without risk of damage. It is to benoted that the time required to cool the coating material isapproximately double that required to heat it to its peak temperature.The use of refrigerated air would allow cooling times to be shortened.

The period of eight seconds for raising a rapid cure material to itspeak temperature is quoted above only by way of example for a typicalrapid cure material. The residence period in the heating section 17 willdepend on the specific rapid cure material being used and, moregenerally, will lie in the range 4 to 60 seconds.

As a conventional oven is designed to provide lengthy transit times inthe heating and cooling sections, such an oven would be unsuitable foruse with rapid cure coating materials. However, in the oven describedwith reference to FIGS. 1 to 16, short transit times may be achieved.This oven is, therefore, suitable for use with rapid cure coatingmaterials. When used with such materials, it is possible to achieve arate of entry of sheets into the oven which is comparable to thatachieved with a conventional oven of equal length using conventionalcoating materials. In the oven 10, the heated air from the plenumchamber 31 or 202 of the conveyor helps to raise the temperature of thesheets rapidly to the curing temperature. Similarly, in the coolingsection, the air from the plenum chamber 88 of the conveyor helps tocool the sheets rapidly to the required cooling temperature.

As is normal in an oven, the oven 10 can cure coating material which hasbeen applied just to the upper surface of sheets 15. However, becausethe upper and lower surfaces of each sheet are free from contact withany part of the oven as they pass therethrough, with a suitablearrangement for transferring sheets from the coating apparatus 12 to theair bed, the oven 10 can also be used to cure coating material which hasbeen applied simultaneously to both surfaces of sheets 15. In order tocoat both surfaces simultaneously, the coating apparatus 12 should beconfigured so that it has two coating cylinders. By applying coatingmaterial to both surfaces of sheets 15 and curing the coating by asingle pass through the oven 10, the number of passes through oven ishalved and the time and energy required to coat and cure the sheets isreduced.

Referring now to FIG. 18, there is shown a modification to the heatingsection 17 of oven 10. In this modification, the plenum chamber 20 isreplaced by three induction heating coils 100, 101, 102 connected to anAC source 103. In a well known manner, the induction heating coilsinduce eddy currents in sheets 15 and thereby cause the temperature inthe sheets to rise to the desired value. The coil 100 will be describedwith reference to FIG. 19.

Referring now to FIG. 19, the coil 100 is shown together with part ofthe conveyor 16. The drive members 35 are shown pushing a sheet 15through coil 100. As may be seen, the bed 30 and drive member 35 alsopass through coil 100. The lower part of coil 100 may pass through theside walls of plenum chamber 31 or beneath plenum chamber 31.

In order to avoid undesirable heating, the bed 30, walls of plenumchamber 31, drive members 35 and chain 36 are all made from anelectrically insulating material.

The coil 100 has five turns 110. Each of the turns 110 comprisesflexible copper conductors 111 which are connected electrically andmechanically at terminals 112, 113. These terminals are connected to asource 114 of alternating current. The frequency of the alternatingcurrent is chosen according to the requirements of the coating materialand metal sheets and will, typically, be in the range 5 kHz to 500 kHz.The power delivered into the metal sheets should be such that thetemperature rise of the sheets does not exceed 200° C./second.

Each of the conductors 111 is enclosed within a flexible pipe 115 formedfrom electrically-insulating plastics material. The pipes 115 are alsoconnected together in terminals 112, 113 and the terminals 112, 113 arethen connected to a source of cooling water.

As may be observed in FIG. 19, the spacing between the conductors 111and the sheets 15 is increased adjacent the side edges. The spacing isincreased in this manner so as to prevent the side edges of sheets 15from achieving a temperature higher than that achieved in their centralparts. The conductors 111 and pipes 115 are held in the position shownin FIG. 19 by a support structure, not shown.

The use of induction coils 100, 101, 102 provides advantages over theplenum chamber 20. By the use of induction heating, the temperature inthe sheet 15 can be elevated more quickly to the required curingtemperature. This helps to reduce the transit time required in the oven10. Also, the use of induction heating ensures that both surfaces of thesheets 15 are raised equally to the required curing temperature. Theinduction coils are particularly suitable for curing both surfaces ofsheets 15 simultaneously. By way of modification, the induction coils100, 101, 102 may be used together with plenum chamber 20.

With the shape shown in FIG. 19, the coil 100 is suitable for use withsteel sheets. As known in the art, if aluminium sheets are to beinduction heated, the sheets are passed between upper and lower coilswhich deliver a transverse flux.

With the induction coils 100, 101, 102, the leading and trailing edgesof the sheets 15 achieve a lower temperature than that achieved in theircentral parts. In some coating materials, this temperature differencemay lead to incomplete curing. With reference to FIGS. 20 and 21, therewill now be described two modified arrangements which overcome thisproblem.

In the modified arrangement of FIG. 20, the induction coil 100 ispreceded by an auxiliary induction heating coil 120. The coil 120 isconnected to an AC source 123. The auxiliary coil 120 is associated witha position detector 124. The detectors 124 detect the passage of theleading and trailing edges of sheets 15 through the coil 100 and thisdetector is connected to a control circuit in the AC source 123. Thecontrol circuit is arranged to energize the coil 120 each time a leadingor trailing edge of one of the sheets 15 passes through it with a shortpulse of high frequency alternating current. A suitable frequency forthis purpose is 100 to 500 kHz. The AC source 123 should operate at ahigher frequency than the AC source 114. The auxiliary heating coilensures that the leading and trailing edges of the sheets 15 achieve atemperature which is close to that achieved in the central parts of thesheets.

The arrangement shown in FIG. 21 is identical to that shown in FIG. 20with one exception. The auxiliary coil 120 and its associated detector124 are placed after coil 120. In both FIGS. 20 and 21, the direction oftravel of the sheets is indicated with an arrow.

In the arrangements shown in FIG. 20, the temperature of each sheet israised in a narrow zone adjacent the leading and trailing edges by thepulses of energy received from the auxiliary coil. As the sheet passesthrough coil 100, heat is conducted away from the narrow zone and a moreeven temperature distribution is achieved. Thus, the arrangement of FIG.20 has this advantage over the arrangement of FIG. 21. However, in thearrangement of FIG. 12, the temperature of a sheet adjacent its leadingand trailing edges may be monitored before each edge passes through coil120 and the energy of the pulse from coil 120 may be adjustedaccordingly.

We claim:
 1. An oven for curing coating material that has been appliedto a plurality of metal sheets, said oven comprising a heating sectionfor applying heat to the metal sheets, wherein the heating sectioncomprises at least one main induction coil, an auxiliary induction coil,first current supplying means for supplying continuous alternativecurrent to the or each main induction coil, and second current supplyingmeans for supplying pulses of alternating current to the auxiliaryinduction coil, the second current supplying means being arranged sothat the auxiliary induction coil receives a current pulse as each ofthe leading and trailing edges of a metal sheet passes therethrough; anda conveyor for conveying the metal sheets through the heating sectionalong a direction of travel, wherein the conveyor comprises: anelongated bed extending through the heating section, a plenum chamberlocated beneath the bed, a matrix of jets formed in the bed forsupplying gas from the plenum chamber to the space above the bed so asto form a gas cushion, the gas cushion being capable of supporting themetal sheets, and means for propelling the metal sheets along the bed.2. An oven for curing coating material that has been applied to aplurality of metal sheets, said oven comprising a heating section forapplying heat to the metal sheets, and a conveyor for conveying themetal sheets through the heating section along a direction of travel,wherein the conveyor comprises: an elongated bed extending through theheating section, a plenum chamber located beneath the bed, a matrix ofjets formed in the bed for supplying gas from the plenum chamber to thespace above the bed so as to form a gas cushion, the gas cushion beingcapable of supporting the metal sheets, and means for propelling themetal sheets along the bed; wherein the propelling means comprises atleast one endless line extending through the heating section, means forimparting motion to said at least one endless line, a first series ofdrive members mounted on said at least one endless line at spaced apartintervals, said first series of drive members being arranged to engagethe leading edges of metal sheets passing along the conveyor, and asecond series of drive members mounted on said at least one endless lineat spaced apart intervals, said second series of drive members beingarranged to engage the trailing edges of metal sheets passing along theconveyor; and wherein said at least one endless line comprises at leastone pair of laterally spaced apart endless lines, the individual drivemembers of said first series of drive members are mounted on, and extendbetween, the two laterally spaced apart endless lines of said pair ofendless lines, and the individual drive members of said second series ofdrive members are mounted on, and extend between, the two laterallyspaced apart endless lines of said pair of endless lines.
 3. An oven forcuring coating material that has been applied to a plurality of metalsheets, said oven comprising a heating section for applying heat to themetal sheets, and a conveyor for conveying the metal sheets through theheating section, wherein the conveyor comprises: an elongated bedextending through the heating section, a plenum chamber located beneaththe bed, a matrix of jets formed in the bed for supplying gas from theplenum chamber to the space above the bed so as to form a gas cushion,the gas cushion being capable of supporting the metal sheets, and meansfor propelling the metal sheets along the bed; wherein the propellingmeans comprises: at least one endless line extending through the heatingsection, means for imparting motion to said at least one endless line, afirst series of drive members mounted on said at least one endless lineat spaced apart intervals, said first series of drive members beingarranged to engage the leading edges of metal sheets passing along theconveyor, and a second series of drive members mounted on said at leastone endless line at spaced apart intervals, said second series of drivemembers being arranged to engage the trailing edges of metal sheetspassing along the conveyor; and wherein said at least one endless linecomprises first and second pairs of endless lines, the individual drivemembers of said first series of drive members are mounted on, and extendbetween, the two laterally spaced apart endless lines of said first pairof endless lines, and the individual drive members of said second seriesof drive members are mounted on, and extend between, the two laterallyspaced apart endless lines of said second pair of endless lines.
 4. Anoven as claimed in any one of claims 2 or 3, in which the individualdrive members of said first and second series of drive members have aV-shape in cross-section.
 5. An oven as claimed in any one of claims 2or 3, further comprising a first endless belt conveyor positionedupstream from said at least one endless line, said first endless beltbeing arranged to bring the metal sheets sheets into engagement with thefirst and second series of drive members, and a second endless beltconveyor extending along a portion of the at least one endless linebetween said first endless belt and the bed, said second endless beltbeing arranged to provide additional support for the metal sheets as themetal sheets pass along said at least one endless line upstream of thebed.
 6. An oven as claimed in claim 1 or or claim 2, in which theheating section includes a second plenum chamber having a lower wallpositioned above the bed of the conveyor, means for supplying heated gasto the second plenum chamber, and means for injecting gas from thesecond plenum chamber to the space below said lower wall.
 7. An oven asclaimed in claim 6, in which the injecting means comprises a set ofslots formed in said lower wall, each slot being oriented so that itsprojection onto the bed of the conveyor is inclined to the direction oftravel of the metal sheets along the conveyor by an angle less than 90°.
 8. An oven as claimed in claim 7, in which the slots are arranged inpairs, the pair of slots being arranged so as to present a V-shapedprofile.
 9. An oven as claimed in claim 2, including means for supplyingheated gas to the plenum chamber located beneath the bed within theheating section.
 10. An oven as claimed in claim 2, including a coolingsection which is located downstream in the direction of travel of theworkpieces from the heating section.
 11. An oven as claimed in claim 2,in which the heating section includes at least one induction coil andmeans for supplying alternating current to the or each induction coil.12. An oven as claimed in claim 2 or claim 10, combined with anapparatus for applying coating material to the metal sheets.